Modern cameras and imaging systems do an excellent job of capturing highly detailed and rich images. They do this efficiently and inexpensively. The problem that remains, however, is discerning what these images have to tell us about the object that is the subject of the image.
In the semiconductor industry, electronic imaging systems are used to inspect or monitor virtually every aspect of the semiconductor device fabrication process. One area of interest is the inspection of bond pads on a semiconductor device, for example a logic circuit (processor) or memory circuit (RAM or ROM). Bond pads are conductive regions on a semiconductor device that are used to connect the semiconductor device to other electronic components. These bond pads are also used to electrically test the semiconductor device once it is has been fabricated. Small electronic probes make contact with the bond pads to connect the newly fabricated device to a tester, a computer that electronically queries the new device to ensure that it operates as desired.
When a probe makes contact with a bond pad, it is desirable to drive the probe into the bond pad to ensure proper electrical contact. If the probe does not make proper contact, the electrical test will fail and it will be unknown whether the device is bad or the electrical contact is simply insufficient. In any case, the end result is generally the same and the device will be scrapped or will at least obtain a much lower price as a second choice or the like. Accordingly, it is important to ensure that a prober, the machine that brings the probes into contact with a semiconductor device's bond pads, operates correctly.
Optical inspection is one approach for determining whether a probe and/or prober is working properly. One inspection option is to optically inspect the probe itself, an option that will not be discussed other than in passing in this application. Another option inspection is to inspect the bond pads on the semiconductor device to see if the probes properly contacted the device. Images of the bond pads of a semiconductor device are captured and inspected to determine whether the probe contacted the bond pad at all, and if so, where. It is also desirable to inspect the marks made by the probes, unimaginatively called probe marks, to determine if the probe had too light a contact, too heavy a contact, or contact that was just right. Given that the geometry of the probe itself is known, the size of the probe mark is indicative of how deep the mark is. It is undesirable for a probe mark to extend into layers of a semiconductor device below the bond pad as this can cause shorting in the device. Further, if too light a contact is made, it is likely that electrical conduction will not take place or will be intermittent.